The prior art contains a number of references to adaptive optics (AO), lenses and systems. The development of AO over the last several years has led to important advances in multiple fields; this has been made possible by virtue of AO's ability to reduce the number of moving parts, foot print and design effort associated with optical systems. One area of AO that has received considerable attention is the liquid lens. In these devices, a volume of fluid typically provides a reconfigurable optical medium. Selected optical properties of the lens are adjusted by manipulating various properties of the fluid and/or the boundary conditions of the compartment or substrates housing the fluid.
Typical liquid lenses fall under one of three categories: electrowetting; liquid crystal (LC); or fluidic. A notable example of an electrowetting lens is provided by Bruno Berge, et al., “Lens with variable focus”, PCT Publication No. WO 99/18456. In that system, a compartment houses two immiscible liquids having different refractive indexes. The interface between the two liquids can vary from substantially flat to substantially spherical in curvature and is largely determined by the contact angle formed between the interface and the wall of the compartment. The curvature of the interface and difference in indexes between the two fluids serves as a lens for light transmitted across the interface. The contact angle will change in response to a voltage applied across the compartment wall. A change in voltage will result in a change in the curvature of the interface and focal power of the lens. Although the electrowetting approach yields a conveniently compact system with low power requirements and fast response, it is difficult to maintain a stable interface for clear apertures greater than about 5-mm diameter.
LC lenses generally utilize the fact that liquid-crystal molecules, which are shaped like tiny rods, can change their orientation in an electric field. Under sufficient field strength, a substantial amount of the molecules can line up parallel to the field. This alters the refractive index, and hence the focal power, of the material. By tailoring the field, substrate and/or LC layer, various optical properties can be controlled. In “Adaptive Liquid Crystal Lenses,” U.S. Pat. No. 6,859,333, Ren, et al., teach a LC lens based on a homogeneous nematic LC layer sandwiched between two transparent substrates. The first substrate includes a spherical or annular ring-shaped Fresnel grooved transparent electrode patterned on its inner surface, while the second substrate includes a transparent electrode coated on its inner surface. When a voltage is applied across the LC layer, a centro-symmetrical gradient distribution of refractive index within the LC layer occurs. The difference in indexes of the LC layer and patterned substrate causes light to focus. By controlling the applied voltage, the focal length of the lens can be tuned continuously. While this device overcomes many limitations typical of other LC lenses, such as strong astigmatism, distortion and light scattering, it suffers from a slow response time. For example, the focusing time of a 6-mm diameter lens having a 40-micron-thick LC layer is approximately 1 second.
The family of fluidic lenses embodies a wide variety of designs and features, however lenses of this type typically comprise the following basic structure: (a) a lens compartment filled with a transparent and incompressible fluid; (b) the compartment is bounded around its sides by a sidewall and on its optical faces by a pair of opposing optical surfaces wherein at least one of the optical surfaces (a “membrane”) is formed from an elastic material and is thus capable of elastic strain; (c) an actuator delivers an actuation force (or “load”, “loading” or “applied load”) to the compartment or fluid, resulting in a pressurization of the fluid and a deformation of the membrane; and (d) once the actuation force is diminished, the restoring or elastic force of the membrane may contribute to the restoration of the membrane to its original or non-actuated state. The change in shape of the membrane and difference in index of refraction between the fluid and medium external to the compartment, result in a change in focal power of the fluidic lens. A system has also been demonstrated (see J. Chen et al., J. Micromech. Microeng. 14 (2004) 675-680) wherein only one lenticular body is provided, bounded on at least one side by an optically clear, compliant membrane. In that system, the refractive power of the lens is controlled by pumping in or out a controlled amount of fluid, thereby changing the curvature of the bounding membrane. That system still suffers from the disadvantage that the pressurized fluid source is located remotely from the compartment. This makes the form-factor of the whole system inconvenient.
While fluidic lenses are capable of overcoming many problems associated with liquid lenses, such as slow response time, instability of large apertures and optical losses, certain limitations remain. For example, in order to reduce the force required by the actuator, it is often desirable for the membrane to be highly compliant and have low elastic (or Young's) modulus, typically in the range of about 0.05 to 2 MPa. However, such low elastic modulus may cause the lens to be susceptible to disturbances, such as instabilities in focus and tilt due to forces of acceleration, and aberrations, such as coma, which may be due to gravitational forces. An approach to mitigating this limitation that has been taught is pre-tensioning of the elastic membrane during lens fabrication, e.g., as described in U.S. Pat. No. 7,697,214 to Robert G. Batchko et al entitled “FLUIDIC LENS WITH MANUALLY-ADJUSTABLE FOCUS”, the entire contents of which are incorporated herein by reference. Pre-tensioning reduces the compliance of the membrane, making it effectively stiffer and thus more resistant to the effects of gravity. Nevertheless, is some instances (for example, lenses with small f/#'s or large apertures) coma and other gravity-induced aberrations persist.
Another inherent disadvantage of many low-elastic modulus membrane materials (for example, polydimethylsiloxane or PDMS) is their permeability, or inability to effectively block the passage of some gases and fluids. Such permeability may result in air bubbles developing in, or fluid leaking out of, the fluidic lens. These effects can diminish the durability, lifetime, optical quality, dynamic range and other performance properties of the lens. Some approaches to solving this problem may include coating the membrane with a high-barrier material or increasing the thickness of the membrane. However, these approaches can result in disadvantageous effects such as increasing the complexity of fabrication, optical scatter and loss, and aberrations.
Yet another inherent disadvantage of typical fluidic lenses is that the shape profile and resulting optical properties of the lens are substantially governed by the tensile elastic properties of the membrane (e.g., Young's modulus, thickness, and amount of pre-tensioning) and fluid pressure. In conventional optics, it is often desirable for the surface of a lens to have a spherical, or prescribed aspheric, profile. However, in the case of fluidic lenses, the highly compliant nature of the membrane generally results in a strong nonlinear dependence of the membrane profile on fluid pressure. Thus, instead of maintaining a spherical profile independent of fluid pressure (i.e., fluid pressure only affecting the radius of curvature), the membrane profile of the fluidic lens may deviate significantly from spherical, with the amount of deviation being dependent on fluid pressure. Such complex dependencies can severely limit the ability to control the optical properties (such as aberrations, the conic constant and other optical effects) of fluidic lenses.
More recently, another co-pending application Ser. No. 12/706,637 (“VARIABLE-FOCAL-LENGTH FLUIDIC LENS WITH REDUCED OPTICAL ABERRATION”, to Batchko et al.) taught means by which the aforementioned limitations could be largely overcome. A key element in overcoming these limitations was the use of membranes constructed from intrinsically stiff materials, such as glass or optical plastics. In such stiff materials the nature of the deformation is substantially a bending strain, whereas in the case of compliant membranes (such as those composed of elastomer films) the deformation is substantially an elastic strain. In such membranes that deform by bending strain, their stiffness is generally sufficient to resist the effects of gravity. However, when the lens profile is no longer afflicted by gravitationally-induced optical coma, spherical aberration (the next higher order aberration commonly associated with fluidic lenses) may now become the dominant aberration. It is well known in the art that spherical aberration is not only associated with fluidic lenses, but also in general with all types of lenses, including static lenses composed of solid materials and other types of adaptive lenses such as liquid crystal and electro-optic lenses. Solutions for providing a fixed (or “static”) spherical correction are known in the art (for example, Schmidt corrector plates). Likewise, dynamic wavefront correction can be accomplished by deformable mirrors [Saito et al., U.S. Pat. No. 7,520,613], liquid crystal spatial light modulators [Barnes et al., U.S. Pat. No. 5,018,838] and mechanical movement of static elements [Alvarez, U.S. Pat. No. 3,305,294 and Simonov et al., WPO International Publication Number WO 2011/019283 A1]. Nonetheless, these solutions suffer from limitations including: high insertion loss; limited range of optical waves of wavefront correction, mechanical complexity and reflective-only (i.e., only non-transmissive or refractive) design.
Despite their low cost and other advantages, the abovementioned limitations and inability to achieve optical performance at a level comparable to that of conventional lenses has thus far prohibited developers from substantially adopting liquid lenses in numerous optical products and applications.
Thus, there is a need in the art for an adaptive optical device that overcomes the above disadvantages.